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:mod:`venv` --- Creation of virtual environments
================================================
.. module:: venv
:synopsis: Creation of virtual environments.
.. moduleauthor:: Vinay Sajip <vinay_sajip@yahoo.co.uk>
.. sectionauthor:: Vinay Sajip <vinay_sajip@yahoo.co.uk>
.. index:: pair: Environments; virtual
.. versionadded:: 3.3
**Source code:** :source:`Lib/venv.py`
--------------
The :mod:`venv` module provides support for creating lightweight "virtual
environments" with their own site directories, optionally isolated from system
site directories. Each virtual environment has its own Python binary (allowing
creation of environments with various Python versions) and can have its own
independent set of installed Python packages in its site directories.
Creating virtual environments
-----------------------------
.. include:: /using/venv-create.inc
.. _venv-def:
.. note:: A virtual environment (also called a ``venv``) is a Python
environment such that the Python interpreter, libraries and scripts
installed into it are isolated from those installed in other virtual
environments, and (by default) any libraries installed in a "system" Python,
i.e. one which is installed as part of your operating system.
A venv is a directory tree which contains Python executable files and
other files which indicate that it is a venv.
Common installation tools such as ``Distribute`` and ``pip`` work as
expected with venvs - i.e. when a venv is active, they install Python
packages into the venv without needing to be told to do so explicitly.
Of course, you need to install them into the venv first: this could be
done by running ``distribute_setup.py`` with the venv activated,
followed by running ``easy_install pip``. Alternatively, you could download
the source tarballs and run ``python setup.py install`` after unpacking,
with the venv activated.
When a venv is active (i.e. the venv's Python interpreter is running), the
attributes :attr:`sys.prefix` and :attr:`sys.exec_prefix` point to the base
directory of the venv, whereas :attr:`sys.base_prefix` and
:attr:`sys.base_exec_prefix` point to the non-venv Python installation
which was used to create the venv. If a venv is not active, then
:attr:`sys.prefix` is the same as :attr:`sys.base_prefix` and
:attr:`sys.exec_prefix` is the same as :attr:`sys.base_exec_prefix` (they
all point to a non-venv Python installation).
API
---
.. highlight:: python
The high-level method described above makes use of a simple API which provides
mechanisms for third-party virtual environment creators to customize environment
creation according to their needs, the :class:`EnvBuilder` class.
.. class:: EnvBuilder(system_site_packages=False, clear=False, symlinks=False, upgrade=False)
The :class:`EnvBuilder` class accepts the following keyword arguments on
instantiation:
* ``system_site_packages`` -- a Boolean value indicating that the system Python
site-packages should be available to the environment (defaults to ``False``).
* ``clear`` -- a Boolean value which, if True, will delete the contents of
any existing target directory, before creating the environment.
* ``symlinks`` -- a Boolean value indicating whether to attempt to symlink the
Python binary (and any necessary DLLs or other binaries,
e.g. ``pythonw.exe``), rather than copying. Defaults to ``True`` on Linux and
Unix systems, but ``False`` on Windows.
* ``upgrade`` -- a Boolean value which, if True, will upgrade an existing
environment with the running Python - for use when that Python has been
upgraded in-place (defaults to ``False``).
Creators of third-party virtual environment tools will be free to use the
provided ``EnvBuilder`` class as a base class.
The returned env-builder is an object which has a method, ``create``:
.. method:: create(env_dir)
This method takes as required argument the path (absolute or relative to
the current directory) of the target directory which is to contain the
virtual environment. The ``create`` method will either create the
environment in the specified directory, or raise an appropriate
exception.
The ``create`` method of the ``EnvBuilder`` class illustrates the hooks
available for subclass customization::
def create(self, env_dir):
"""
Create a virtualized Python environment in a directory.
env_dir is the target directory to create an environment in.
"""
env_dir = os.path.abspath(env_dir)
context = self.create_directories(env_dir)
self.create_configuration(context)
self.setup_python(context)
self.setup_scripts(context)
self.post_setup(context)
Each of the methods :meth:`create_directories`,
:meth:`create_configuration`, :meth:`setup_python`,
:meth:`setup_scripts` and :meth:`post_setup` can be overridden.
.. method:: create_directories(env_dir)
Creates the environment directory and all necessary directories, and
returns a context object. This is just a holder for attributes (such as
paths), for use by the other methods.
.. method:: create_configuration(context)
Creates the ``pyvenv.cfg`` configuration file in the environment.
.. method:: setup_python(context)
Creates a copy of the Python executable (and, under Windows, DLLs) in
the environment.
.. method:: setup_scripts(context)
Installs activation scripts appropriate to the platform into the virtual
environment.
.. method:: post_setup(context)
A placeholder method which can be overridden in third party
implementations to pre-install packages in the virtual environment or
perform other post-creation steps.
In addition, :class:`EnvBuilder` provides this utility method that can be
called from :meth:`setup_scripts` or :meth:`post_setup` in subclasses to
assist in installing custom scripts into the virtual environment.
.. method:: install_scripts(context, path)
*path* is the path to a directory that should contain subdirectories
"common", "posix", "nt", each containing scripts destined for the bin
directory in the environment. The contents of "common" and the
directory corresponding to :data:`os.name` are copied after some text
replacement of placeholders:
* ``__VENV_DIR__`` is replaced with the absolute path of the environment
directory.
* ``__VENV_NAME__`` is replaced with the environment name (final path
segment of environment directory).
* ``__VENV_BIN_NAME__`` is replaced with the name of the bin directory
(either ``bin`` or ``Scripts``).
* ``__VENV_PYTHON__`` is replaced with the absolute path of the
environment's executable.
There is also a module-level convenience function:
.. function:: create(env_dir, system_site_packages=False, clear=False, symlinks=False)
Create an :class:`EnvBuilder` with the given keyword arguments, and call its
:meth:`~EnvBuilder.create` method with the *env_dir* argument.
|
